1. Field of the Invention
The present invention relates to a stacked layer structure containing a light-emitting layer and having a quantum well structure including a barrier layer formed from a boron-containing Group III-V compound semiconductor.
2. Background Art
In general, the term xe2x80x9cquantum well (QW) structurexe2x80x9d refers to a superlattice structure in which thin barrier layers and well layers are stacked alternately and periodically (see xe2x80x9cFundamentals of Semiconductor Laserxe2x80x9d edited by The Japan Society of Applied Physics, first printing, first edition, published by Ohmsha, Ltd. on May 20, 1987, pp. 140-146). The barrier layer serves as a layer for localizing carriers in the well layer, and the barrier layer is generally formed from a semiconductor material having a bandgap larger than that of a semiconductor material constituting the well layer (see the aforementioned xe2x80x9cFundamentals of Semiconductor Laser,xe2x80x9d page 140). A quantum well structure including only one well layer is called a xe2x80x9csingle quantum well (SQW) structure,xe2x80x9d and a quantum well structure including a plurality of periodically stacked well layers is called a xe2x80x9cmulti quantum well (MQW) structurexe2x80x9d (see the aforementioned xe2x80x9cFundamentals of Semiconductor Laser,xe2x80x9d page 171). Conventionally, there has been disclosed a technique for forming a field effect transistor having a quantum well structure which can be operated at high speed by means of electrons which are localized in a well layer under the barrier effect of a barrier layer (see U.S. Pat. No. 4,163,237).
A technique has been known for forming a light-emitting device which emits light of short wavelength, such as a light-emitting diode (LED) which emits blue light or green light, or a laser diode (LD), which includes a light-emitting layer (active layer) having a quantum well structure (see xe2x80x9cGroup III Nitride Semiconductor,xe2x80x9d first edition, published by Baifukan Co., Ltd. on Dec. 8, 1999, pp. 247-252). Conventionally, in a single or multi quantum well structure formed of a well layer and a barrier layer, the well layer has generally been formed from gallium indium nitride (GaXIn1xe2x88x92XN: 0xe2x89xa6Xxe2x89xa61) (see U.S. Pat. No. 6,153,894), and the barrier layer has generally been formed from aluminum gallium nitride (AlXGa1xe2x88x92XN: 0xe2x89xa6Xxe2x89xa61) (see U.S. Pat. No. 6,153,894).
A light-emitting layer having a single quantum well structure or multi quantum well structure has been provided conventionally on a cladding layer formed from a Group III nitride semiconductor such as p-type or n-type gallium nitride (GaN). Recently, a light-emitting diode (LED) including a substrate formed from single crystal silicon (Si), a cladding layer formed from a boron phosphide semiconductor, and a light-emitting layer provided on the cladding layer has been proposed.
Conventionally, a light-emitting diode including a light-emitting layer formed from gallium indium nitride (GaXIn1xe2x88x92XN: 0xe2x89xa6Xxe2x89xa61) has been disclosed (see Japanese Patent Publication (kokoku) No. 55-3834), and a light-emitting diode including a light-emitting layer formed from a boron-containing Group III-V compound semiconductor of a multi-component mixed crystal has also been disclosed (see Japanese Patent Application Laid-Open (kokai) No. 10-247745). For example, a light-emitting layer having a superlattice structure containing Ga0.25Al0.3B0.5N0.5P0.5 (i.e., a pentanary mixed crystal) and boron phosphide (BP) are known (see Japanese Patent Application Laid-Open (kokai) No. 10-247745).
When a light-emitting layer is formed of a superlattice-structure layer containing a boron-containing Group III-V compound semiconductor of a multi-component mixed crystal as described above, a special growth apparatus is required for attaining abrupt changes of composition at a junction interface between layers constituting a superlattice structure (see Japanese Patent Application Laid-Open (kokai) No. 2-288371). When the number of constitutive elements is reduced, a mixed crystal of a stable composition can be obtained more advantageously and conveniently (see xe2x80x9cAn Introduction to Semiconductor Devicexe2x80x9d authored by Iwao Teramoto, first edition, published by Baifukan Co., Ltd. on Mar. 30, 1995, page 24). In order to successfully obtain the aforementioned pentanary mixed crystal in which compositions of constitutive elements are constant, a sophisticated formation technique for, e.g., precisely adjusting the feed amounts of the elements is required, making the operation troublesome.
In the case of a conventional structure including a cladding layer formed from a boron phosphide Group III-V compound semiconductor and a light-emitting layer formed of a single layer of, for example, gallium indium nitride, the light-emitting layer being provided on the cladding layer, the half width of the wavelength of light emitted from the light-emitting layer is broad. Therefore, there has been demand for a light-emitting layer which emits light of high monochromaticity. The half width of the central wavelength (about 420 nm) of violet light emitted from a light-emitting layer formed from gallium indium nitride (GaXIn1xe2x88x92XN) (i.e., a conventional light-emitting layer) is as large as about 380 meV to about 400 meV.
When a light-emitting layer is constructed to have a quantum well structure rather than being formed to have a single layer, the light-emitting layer is well known to emit monochromatic light by virtue of attainment of uniform quantum level (see the aforementioned xe2x80x9cFundamentals of Semiconductor Laser,xe2x80x9d page 164). Also, when a light-emitting layer constituting a laser diode (LD) is formed of a quantum well structure, the threshold voltage (i.e., Vth) of the LD can be reduced (see the aforementioned xe2x80x9cFundamentals of Semiconductor Laser,xe2x80x9d pp. 173-178). However, it has not been known so far that a light-emitting layer of quantum well structure can be readily joined to a boron-containing Group III-V compound semiconductor layer and where the light-emitting layer can emit light of high monochromaticity.
In view of the foregoing, an object of the present invention is to provide a stacked layer structure comprising a single crystal substrate; an amorphous or polycrystalline buffer layer formed from a Group III-V compound semiconductor containing boron (B) (i.e., a boron-containing Group III-V compound semiconductor), the buffer layer being provided on the substrate; a cladding layer formed from a boron-containing Group III-V compound semiconductor, the cladding layer being provided on the buffer layer; and a light-emitting layer formed from a Group III-V compound semiconductor, the light-emitting layer being provided on the cladding layer. Another object of the present invention is to provide a light-emitting device comprising the stacked layer structure, which emits light of high monochromaticity. Yet another object of the present invention is to provide a lamp comprising the light-emitting device. A further object of the present invention is to provide a light source unit comprising the lamp.
A first aspect of the present invention, provides a stacked layer structure comprising a single crystal substrate; an amorphous or polycrystalline buffer layer formed from a Group III-V compound semiconductor containing boron (B) (i.e., a boron-containing Group III-V compound semiconductor), the buffer layer being provided on the substrate; a cladding layer formed from a boron-containing Group III-V compound semiconductor, the cladding layer being provided on the buffer layer; and a light-emitting layer having a quantum well structure including a barrier layer formed from a boron-containing Group III-V compound semiconductor and a well layer formed from a Group III nitride semiconductor, the light-emitting layer being provided on the cladding layer, wherein the barrier layer is formed from a boron-containing Group III-V compound semiconductor having the same lattice constant as the boron-containing Group III-V compound semiconductor constituting the cladding layer.
Since the light-emitting layer has the aforementioned quantum well structure, the layer emits light of high monochromaticity.
Preferably, the well layer is formed from a Group III nitride semiconductor having the same lattice constant as the boron-containing Group III-V compound semiconductor constituting the cladding layer.
Preferably, the well layer is formed from a semiconductor having the same lattice constant as the semiconductor constituting the barrier layer.
Preferably, the barrier layer is formed from boron gallium indium phosphide (BXGaYIn1xe2x88x92Xxe2x88x92YP: 0xe2x89xa6Xxe2x89xa61, 0 less than Y less than 1, 0 less than X+Yxe2x89xa61).
Preferably, the buffer layer is formed from a boron-containing Group III-V compound semiconductor having a composition capable of establishing lattice matching with a single crystal material constituting the substrate.
Preferably, the cladding layer is formed of a boron-containing Group III-V compound semiconductor layer, and has a first surface that faces the buffer layer having a boron composition capable of establishing lattice matching with the buffer layer, and a second surface that faces the light-emitting layer having a boron composition capable of establishing lattice matching with the barrier layer or the well layer, wherein a boron compositional proportion is graduated in a thickness direction.
Preferably, the cladding layer is formed from boron gallium indium phosphide (BXGaYIn1xe2x88x92Xxe2x88x92YP: 0 less than Xxe2x89xa61, 0xe2x89xa6Y less than 1, 0 less than X+Yxe2x89xa61).
Preferably, an intermediate layer formed from a Group III-V compound semiconductor is provided between the cladding layer and the light-emitting layer, the semiconductor containing an element which constitutes a Group III-V compound semiconductor constituting the light-emitting layer.
Since the intermediate layer is provided under the light-emitting layer, the light-emitting layer contains no microcracks; i.e., exhibits excellent continuity.
Preferably, the intermediate layer is formed from a Group III-V compound semiconductor capable of establishing lattice matching with a Group III-V compound semiconductor constituting the light-emitting layer.
Preferably, the intermediate layer is formed from the same Group III-V compound semiconductor as that constituting the light-emitting layer.
A second aspect of the present invention provides a light-emitting device comprising any of the stacked layer structures of the present invention.
The light-emitting device comprising the stacked layer structure emits light of high intensity and high monochromaticity.
A third aspect of the present invention provides a lamp comprising the light-emitting device.
The lamp comprising the light-emitting device emits light of high intensity and high monochromaticity.
A fourth aspect of the present invention provides a light source unit comprising the lamp.
The light source unit comprising the lamp emits light of high intensity and high monochromaticity.